Many labeled packages and/or containers are subjected, after removal of their contents, to cleaning (or recycling) treatments either for the purpose of reusing them or, after destruction, for the purpose of recovering their constituent material. Such treatments often require the label to be completely separated from the article to which it is fastened, without leaving adhesive residues on the surface of said article, so as to make the recycling process easier.
These recycling processes, such as the one used for cleaning spent glass bottles, for example beer bottles, generally include a step of immersing the article to be cleaned in basic aqueous compositions maintained at temperatures between 60 and 100° C. The purpose of such a step is to debond the label fastened to the article and to separate it therefrom.
Pressure-sensitive adhesives or PSAs are substances giving the support coated therewith a tack at room temperature, which allows its instantaneous adhesion to a substrate under the effect of brief slight pressure. PSAs are widely used for the manufacture of self-adhesive labels, which are fastened to articles for the purpose of presenting information (such as a barcode, description, price) and/or for decorative purposes.
PSAs are generally applied by continuous coating processes to the entire surface of a printable support layer of large dimensions, consisting of paper or a film of polymer material having one or more layers. The adhesive layer that covers the printable support layer (generally the side opposite the printable side) is itself covered with a protective layer (often called a “release liner”), consisting for example of a siliconized film. The multilayer system obtained is generally packaged by being wound up in the form of large reels up to 2 m in width and 1 m in diameter, which can be stored and transported.
This multilayer system can be subsequently converted into self-adhesive labels that can be applied by the end user by means of conversion processes that include the printing of desired decorative elements and/or information on the printable side of the support layer, followed by cutting to the desired shape and dimensions. The protective layer may be easily removed without modifying the adhesive layer that remains fastened to the printable support layer. After separation from its protective layer, the label is applied at a temperature close to the ambient temperature to the article to be coated, either manually or with the use of labellers on automated packaging lines.
PSAs, because of their high room-temperature tack, make it possible for the label to be rapidly bonded to the article to be coated (for example bottles), suitable for obtaining high industrial production rates.
U.S. Pat. No. 3,763,117 describes an acrylate-based PSA having good adhesive properties, which allows easy separation when hot using a basic aqueous solution.
U.S. Pat. No. 5,385,965 also describes a PSA making it possible, after coating on a paper support or film of a polymer material, to obtain a label that can be detached from the substrate through the action of a hot alkaline solution. This PSA takes the form of an aqueous emulsion of an acrylate-based copolymer or of a styrene-butadiene copolymer. The solids content of this emulsion does not exceed 70%, so that coating this PSA on the support layer is complicated by the need for an emulsion drying step. In addition, such adhesives have the drawback of passing partially into a solution or into suspension in the aqueous compositions used for recycling labeled bottles, thereby imposing on industrial installations constraints in the reprocessing of these compositions before discharge into the environment.
U.S. Pat. No. 4,680,333 teaches a hot-melt pressure-sensitive adhesive composition comprising a styrene/isoprene/styrene block copolymer, an aliphatic resin having a low softening temperature and a metal salt of a fatty acid. This composition makes it possible, after a paper support has been coated with it and this support has been bonded to a substrate, to debond the thus fastened support at any moment, dry and at room temperature, without any other treatment, thus giving the coated support a repositionable adhesive behavior.
Hot-melt adhesives or hot melts (HMs) are substances that are solid at room temperature and contain neither water nor solvent. They are applied in the melt state and solidify upon being cooled, thus forming a joint for fastening the substrates to be assembled. Certain hot melts are formulated so as to give the support coated therewith a relatively hard and tack-free character. Other hot melts provide the support with a relatively soft and high-tack character. PSAs are widely used for the manufacture of self-adhesive labels—the corresponding adhesives are denoted by hot-melt pressure-sensitive adhesives or HMPSAs.
The HMPSA composition described by the U.S. Pat. No. 4,680,333 does not allow the label coated therewith to bond permanently to a substrate, since said label may be debonded dry at any moment.
Patent application US 2004/0220308 describes a hot-melt adhesive composition comprising a styrene block copolymer, an acid functional diluent, such as a fatty acid, and an acid tackifying resin having an acid number of greater than 100.